The present invention is directed generally to bonding ceramic articles. It is directed in particular to a method for isothermally bonding ceramic materials and refractory metals.
Ceramics have low specific gravities and high flexure and tensile strengths. They therefore have potential applications in heat engines and heat exchangers, as well as microelectronics and a variety of other technologies. There is thus a need for reliable and efficient methods of bonding ceramics to ceramics. Particularly, for engine applications, the bond between the two ceramic articles must be able to withstand high amounts of stress and high temperatures. Bonding a ceramic to a ceramic or a ceramic to a metal is frequently accomplished using a metal interlayer between the materials to be bonded. Well-known bonding methods include solid-state bonding and brazing.
Brazing involves holding articles to be bonded in contact with a metal interlayer to form an assembly. The assembly is heated to a temperature sufficient to melt the interlayer. The interlayer preferably includes an alloy which forms a reactive liquid phase for wetting the articles to be bonded. After the interlayer has been melted, the assembly may be cooled to solidify the interlayer and form a bond.
Materials used to form a brazing interlayer include pure aluminum (Al) and Al alloys, silver-titanium (Ag-Ti) alloys, silver-copper-titanium (Ag-Cu-Ti) alloys, silver-copper-indium-titanium alloys (Ag-Cu-In-Ti) alloys, silver-copper-tin-titanium alloys (Ag-Cu-Sn-Ti) alloys and copper-nickel-titanium (Cu-Ni-Ti) alloys. Although more refractory brazing alloys exist, the most commonly used brazing alloys have brazing temperatures below about 1000.degree. C. As such, assemblies joined by such brazing alloys are unlikely to be suitable for high temperature applications. Further, even for low temperature applications, the expansion coefficient of a brazing interlayer and a ceramic may be sufficiently different that significant thermal stresses may result when an assembly is heated or cooled. The thermal stresses may result in immediate fracture of the assembly, or ultimate failure if the assembly is subjected to thermal cycling.
Ceramics are preferably bonded using an interlayer including a refractory metal. Refractory metals are preferred not only because of their high melting point, but because they include metals which have the closest expansion coefficient match with ceramics. A reasonably good expansion match is important in reducing thermal stresses when a bond is cooled from its bonding temperature, or when a bond is to be used in thermal cycling conditions.
Solid-state bonding may be used when bonding ceramic materials using such a refractory metal interlayer. Solid-state bonding involves holding articles to be bonded in contact with a metal interlayer to form an assembly. The assembly is held at a temperature greater than about half the melting point of the interlayer, and subjected to a relatively high pressure, for example, greater than about ten Mega Pascals (10 MPa), for a prolonged period until the assembly is bonded. In solid-state bonding, the surfaces of the articles to be bonded should be meticulously clean. It is also desirable to make the surfaces very flat, preferably polished, to ensure the optimum contact with the bonding interlayer. Although a solid-state bond may be relatively strong, time required for surface preparation, the relatively high bonding pressure, and relatively high temperatures, may make the method unsuitable for mass production.
A disadvantage of high temperature methods for providing a ceramic-to-ceramic, metal-to-ceramic or metal-to-metal bond, particularly methods which require prolonged exposure of articles being bonded to a relatively high temperature, is that it may cause undesirable microstructural changes in the article during the bonding process.
A paper by Duvall et al., "TLP Bonding: a New Method for Joining Heat Resistant Alloys", Welding Journal, Vol 53, No 4, pp. 203-214, April 1974, discloses a method for bonding high-temperature metal alloy articles at a relatively low temperature. The method includes forming a bonding interlayer including an alloy. The bonding layer alloy includes an alloy similar to the alloy of the articles to be bonded, but also includes a relatively small percentage of material, for example, boron, for lowering the melting point of the bonding interlayer alloy. The low melting point alloy interlayer is placed between the articles to be bonded to form an assembly. The assembly is raised to a bonding temperature above the melting point of the interlayer and the entire interlayer melts to form a liquid. The liquid interlayer is said to comprise a transient liquid phase (TLP) of the alloy from which it is formed. The assembly is held at the bonding temperature and the melting point reducing material diffuses out of the TLP alloy into the adjoining articles. As the melting point reducing material diffuses out of the liquid the interlayer region solidifies to form a bond. After the bond has solidified it is preferably held at the bonding temperature for several hours to homogenize the interlayer in composition and structure. During the homogenizing process more of the melting point reducing material may diffuse from the interlayer alloy further increasing the melting point of the interlayer. As such, a bonded assembly may be used at a temperature higher than the original bonding temperature. The TLP bonding method of Duvall et al is advantageous in that the bonding interlayer actually melts at the onset of the bonding process. Melting the interlayer may enable it to fill an irregular space between articles being bonded. Melting the interlayer also eliminates the need to apply high pressure to the articles during bonding. As such, the method may be used without meticulous surface preparation, and may be used to bond irregularly shaped articles, i.e., for bonding non-flat surfaces.
While the method of Duvall et al. may be effective for bonding together metal alloy articles, it does not appear to be suitable for forming a bond to a ceramic article. This is because the method depends on diffusing a melting point lowering component out of an alloy interlayer and into an adjacent metal article. Accordingly, it is an object of the present invention to provide a TLP bonding method suitable for forming a metal to ceramic or a ceramic-to-ceramic bond.
It is another object of the invention to provide a metal-ceramic or a ceramic-ceramic bonded assembly which may be used at a higher temperature than the temperature at which it was bonded.